This invention relates generally to marine propulsion devices such as outboard motors and stern drive units and, more particularly, to exhaust gas discharge systems including idle exhaust gas release arrangements.
A properly designed exhaust system provides a means for an internal combustion engine to exhaust or vent combustion carbon products to atmosphere. In many circumstances, outboard motors vent their exhaust gases downwardly through an exhaust housing to a through-the-hub propeller. At high speeds or when the motor is raised up in the water so that the hub is near the surface of the water, exhaust gases can easily pass through the exhaust housing and out through the propeller hub. At idle or slow speed conditions, however, the outboard motor sets lower in the water resulting in an increased back pressure at the propeller hub. This increased back pressure can restrict or prevent the venting or exhaust gases.
Idle exhaust relief systems have accordingly been provided for venting of exhaust gases at idle or slow speed. Such systems have provided for direct contact of an exhaust relief passageway with a wall of water provided for cooling of the exhaust gas duct running to the propeller hub. When the exhaust relief system is in unrestricted communication with this wall of water, the amount of back pressure from the wall of water will vary with boat load conditions, water conditions, and the overall system design of the wall of water. This back pressure caused by the wall of water can significantly reduce the efficiency of an idle exhaust relief system which is in unrestricted communication with the wall of water.
Examples of prior constructions which include exhaust relief systems are Sanmi et al. U.S. Pat. No. 4,303,401, Kollman U.S. Pat. No. 3,310,222 and Maier et al. U.S. Pat. No. 4,036,162. An exhaust relief system which is not in communication with a wall of water but which is different than the following invention is disclosed in Miller et al. U.S. Pat. No. 3,911,852.